1. Field of the Invention
Embodiments of the invention relates to an LED drive circuit which converts an alternating current commercial power source to a direct current voltage necessary to cause light emitting diodes (hereafter referred to as “LEDs”) to emit light, and controls lighting of the LEDs, and particularly, relates to an LED drive circuit wherein the need for an auxiliary winding of a transformer, a current limiting resistor thereof, a rectifier diode, a smoothing capacitor, or the like, which has heretofore been provided in order to supply the power source to a control circuit in the LED drive circuit is eliminated.
2. Related Art
Among lighting units using an LED bulb, there is a lighting unit which is lighted using a commercial power source. When lighting of this kind of lighting unit is controlled, a flyback converter configuration LED drive circuit is generally used to generate a direct current voltage which drives LEDs using the commercial power source and an AC-DC converter.
FIG. 6 is a circuit diagram showing one example of a heretofore known flyback converter configuration LED drive circuit.
The LED drive circuit shown here is a circuit for generating a desired direct current output voltage Vout from a commercial power source voltage of an AC power source 1 and driving LEDs 51 to 53. The LED drive circuit is configured including a diode bridge rectifier circuit 2, an alternating current switch 3, an inductor Lin and capacitor C1 configuring an input filter circuit, two smoothing capacitors C2 and C3, a phase compensation capacitor Ccomp, a switching element M1, a drive IC circuit 4 which on/off controls the switching element M1, a sensing resistor R1, a current sensing resistor R2, a current limiting resistor R3, two rectifier diodes D1 and D2, and a transformer T.
Also, an internal circuit configuration of the drive IC circuit 4 is shown in FIG. 7. The drive IC circuit 4 includes a VCC terminal 41, a VH terminal 42, an IS terminal 43, an FB terminal 44, a COMP terminal 45, an OUT terminal 46, and a GND terminal 47, and a main circuit section of the drive IC circuit 4 is mainly configured including a start-up circuit 10 and a pulse control circuit 20.
The start-up circuit 10 is configured of a low-voltage malfunction protection circuit (hereafter referred to as an UVLO) 11, a reference voltage source 12 which supplies two differing threshold voltages to the UVLO 11 as an UVLO cancel voltage Vref1 and UVLO voltage Vref2 (note that Vref1>Vref2), a junction field effect transistor (JFET) 13, a current source circuit 14 which generates a starting current Ist, a switch 15, and an inverter circuit 16.
Herein the drain of the junction field effect transistor (hereafter referred to as the JFET) 13 is connected to the high-voltage VH terminal 42, and the source is connected to one end of the switch 15 via the current source circuit 14. Also, the gate of the JFET 13 is connected to the ground. The JFET 13 is such that the higher the source potential thereof than the gate potential, the lower a drain current. Consequently, when the source potential is too high, only a current still lower than a current defined by the current source circuit 14 flows.
The pulse control circuit 20 is configured of an error amplifier 21, a reference voltage source 22, a comparator 23, an RS flip flop 24 (hereafter referred to as an RSFF), an oscillator 25, an AND circuit 26, and a buffer amplifier 27. In the pulse control circuit 20, the RSFF 24 is set by a fixed-cycle pulse signal output from the oscillator 25. Also, a signal (an error signal) wherein a difference between a voltage signal corresponding to a load level received at the FB terminal 44 and a reference voltage Vref output from the reference voltage source 22 is amplified by the error amplifier 21 is compared by the comparator 23 with a current sensing voltage signal Vs input into the IS terminal 43, and when the voltage signal reaches the error signal, the RSFF 24 is reset. An output signal (a Q output) from an output terminal Q of the RSFF 24 is output from the OUT terminal 46 as a pulse width modulated pulse signal via the AND circuit 26 and buffer amplifier 27. In this way, the drive IC circuit 4 is configured in such a way as to on/off control the switching element M1 using the pulse signal from the OUT terminal 46 (the switching element M1 is turned on when the Q output of the RSFF 24 is at an H (high) level).
Returning to FIG. 6, in the LED drive circuit, when the alternating current switch 3 is turned on, the commercial power source voltage of the AC power source 1 is rectified and applied to the input side of the transformer T. In the drive IC circuit 4, as the terminal voltage of the capacitor C1 connected to the high-voltage VH terminal 42 rises at this time, the starting current Ist flows from the VH terminal 42 to the VCC terminal 41 via the start-up circuit 10 which is an internal circuit of the drive IC circuit 4. That is, the start-up circuit 10 has a function of a current supply circuit which supplies current to the capacitor which starting. Because of this, it is possible to start charging of the smoothing capacitor C3 connected to the VCC terminal 41.
The drive IC circuit 4 is such that when the voltage of the VCC terminal 41 reaches the UVLO cancel voltage Vref1, of the threshold voltages of the UVLO 11, the switch 15 in the start-up circuit 10 is turned off, and the starting current Ist flowing from the VH terminal 42 to the VCC terminal 41 can be stopped.
At the same time with this, the signal from the start-up circuit 10 supplied to the AND circuit 26 switches from a previous L (low) level to an H (high) level, and the output signal of the OUT terminal 46 becomes able to be on/off controlled in response to the Q output signal of the RSFF 24. That is, the switching element M1 of the LED drive circuit, on receiving the output signal from the OUT terminal 46 of the drive IC circuit 4, repeats an on/off operation.
The switching element M1 is provided on a primary coil L1 side of the transformer T, and a voltage based on the input voltage Vin supplied to the primary coil L1 is induced on a secondary coil L2 side by the on/off operation of the switching element M1. Consequently, the voltage induced in the secondary coil L2 of the transformer T is rectified and smoothed by the secondary side rectifier diode D2 and smoothing capacitor C2, and becomes the direct current output voltage Vout, and the direct current output voltage Vout is applied to the plurality of LEDs 51 to 53 connected in series.
Herein, in order to stably cause the LEDs 51 to 53 to emit light with the direct current output voltage Vout, it is necessary to control a current flowing thereinto to be constant. The switching element M1 is turned on by the RSFF 24 being set by the signal from the oscillator 25 built into the drive IC circuit 4, and the output of the OUT terminal 46 changing from the L level to the H level. Also, the load current 10 from the secondary coil L2 and smoothing capacitor C2 flows to the LEDs 51 to 53, but is voltage converted by the current sensing resistor R2, and input into the FB terminal 44 of the drive IC circuit 4. Also, the voltage level of the COMP terminal 45 is determined by a degree of error between the voltage signal of the FB terminal 44 and the reference voltage Vref of the reference voltage source 22. That is, the error amplifier 21 being a trans-conductance amplifier, a current corresponding to the difference between the voltage signal of the FB terminal 44 and the reference voltage Vref of the reference voltage source 22 flows from the error amplifier 21 to the capacitor Ccomp connected to the COMP terminal 45, and the voltage level of the COMP terminal 45 is determined by the current being integrated by the capacitor Ccomp.
Further, when the voltage signal Vs of the IS terminal 43 reaches the voltage level of the COMP terminal 45, the RSFF 24 is reset, the output from the OUT terminal 46 changes from the H level to the L level, and the switching element M1 is turned off. Herein, when the load current 10 flowing to the LEDs 51 to 53 is lower than a set value, the on-duty of the switching element M1 becomes wider, and when the current flowing to the LEDs 51 to 53 is higher than the set value, the on-duty of the switching element M1 becomes narrower. In other words, the drive IC circuit 4 operates in such a way as to cause a certain magnitude of load current 10 to flow to the secondary coil L2 side of the transformer T, and a duty control is carried out on the LEDs 51 to 53. See Japanese patent application no. JP-A-2011-35112 (also referred to herein as “PTL 1”).
The feature of this kind of heretofore known LED drive circuit is in that when the power source voltage is supplied to the drive IC circuit 4 of the LEDs 51 to 53, a coil L3 configuring an auxiliary winding with the same polarity as the secondary coil L2 is added to the transformer T, and the coil L3 is connected to the VCC terminal 41 of the drive IC circuit 4. That is, as heretofore described, when the voltage of the VCC terminal 41 reaches the UVLO cancel voltage Vref1, the switch 15 in the start-up circuit 10 is turned off, thus stopping the starting current Ist flowing from the VH terminal 42 to the VCC terminal 41, and subsequently, power is supplied to the drive IC circuit 4 by an electromotive force generated in the coil L3 configuring the auxiliary winding by the switching operation of the switching element M1. Further, the rectifier diode D1 and smoothing capacitor C3 are necessary in order to connect the coil L3 to the VCC terminal 41. Also, the voltage value of the VCC terminal 41 is determined by the turn ratio of the secondary coil L2 and coil L3, but the current limiting resistor R3 must be interposed in series with the rectifier diode D1 in order for the voltage at the VCC terminal 41 not to rise due to a surge voltage generated on the primary side of the transformer T.
With the heretofore known LED drive circuit shown in FIG. 6, in order to stably supply a power source to the drive IC circuit 4 after the start-up, not only is the auxiliary winding provided in the transformer T, apart from the drive IC circuit 4, but the rectifier diode D1, current limiting resistor R3, smoothing capacitor C3, and the like, must be additionally incorporated.
However, with regard to an LED bulb, it is necessary to fit the LED drive circuit, as well as the LED main body, in a bulb of the same size as a heretofore known incandescent bulb, and it is in general difficult to fit the LED drive circuit with a large number of parts in a limited space of the LED bulb, meaning that it is important to reduce the number of parts configuring the LED drive circuit.
FIG. 8 is a diagram showing another example of the heretofore known LED drive circuit.
In this LED drive circuit, a supply of power source to the kind of drive IC circuit 4 shown in FIG. 7 (a supply of current to a capacitor C4 connected to the VCC terminal 41) is constantly carried out via the start-up circuit 10 connected to the VH terminal 42. Because of this, it is not necessary to provide an auxiliary winding in the transformer T.
However, with this LED drive circuit, in order to stabilize the voltage supplied to the VCC terminal 41 which is a power source terminal, the high capacity capacitor C4 is necessary between the VCC terminal 41 and GND terminal 47. Also, as an arrangement is such that an operating current of the drive IC circuit 4 is supplied by supplying the high-voltage input voltage Vin directly to the VH terminal 42, there is a problem in that power consumed in the drive IC circuit 4 becomes larger.
Japanese patent application no. JP-A-2008-278640 (also referred to herein as “PTL 2”) discloses an invention of a power supply and lighting unit wherein it is possible to obtain a stable internal power source, and it is possible to omit parts such as an auxiliary winding, by making the anode voltages of LEDs and the power source of a drive IC circuit common to each other.
However, with the configuration of the power supply disclosed here, no power source is supplied to a control circuit (53) unless a switching of a switching transistor (33) starts. Meanwhile, the switching of the switching transistor (33) cannot be started unless power is supplied to the control circuit (53). Consequently, there occurs a problem in that the power supply with this kind of configuration cannot start the drive IC circuit.
Also, Japanese patent application no. JP-A-2009-232624 (also referred to herein as “PTL 3”) discloses an invention of a power supply and lighting unit wherein it is possible to stably light a semiconductor light emitting element by adopting a configuration such that the power source of a control circuit is supplied from the secondary side of a transformer to a drive IC circuit.
In PTL 3, as a start-up controller (28) is provided so as to enable a drive IC circuit to start, the problem of PTL 2 is solved. Herein, the start-up controller (28) is connected to a primary winding (14a) of a switching transformer (14) and, when the output of a ripple current smoothing capacitor (13) is supplied to the primary winding (14a) of the switching transformer (14) by a power source being applied, supplies a starting output to a control circuit (26) for a predetermined time. That is, the start-up controller (28), when starting, supplies power to the control circuit (26) for the predetermined time. Meanwhile, in order to cause a switching transistor (15) to switch, it is necessary to instantaneously cause a current of several hundred mA to flow in order to charge and discharge the gate capacitance of the switching transistor (15).
Consequently, the start-up controller (28) has to be able to turn on/off a current of several hundred mA, and when the start-up controller (28) is attempted to be configured of a semiconductor element, the chip size thereof is large. Also, when the chip size of the start-up controller (28) is attempted to be reduced, it is necessary to prepare a buffer capacitor, apart from the reduction in size. Moreover, a high capacity capacitor is required in order to instantaneously supply a current of several hundred mA from the buffer capacitor. In this way, with the invention of PTL 3, a cost problem occurs newly for an LED bulb power supply or that kind of lighting unit.